A conventional vortex pneumatic classifier disperses with air flow particulate raw material, for example, granular or powdered material such as limestone dust, classifies the said granular or powdered material into coarse powder and fine powder employing the balance between centrifugal force and drag force, and at the same time, discharges the said fine powder to the exterior of the machine, which then becomes product. (See Japanese Patent Publication No. 57-24189.)
As is generally known, in the event that the theoretical classifying particle diameter Dp(th) [m] is where the particle Reynolds number Rep=Dp(th) Vr .rho.f/.mu.&lt;2, it can be obtained by the general formula described below. ##EQU1##
In this general formula, Vt indicates the peripheral speed (m/s) of the tip of the vortex flow adjusting vanes, .mu. indicates the viscosity coefficient of the air (Pa.S), D indicates the rotor diameter (m), Vr indicates the speed of the inwardly flowing air (m/s) at the tip of the vortex flow adjusting vanes, and .rho.p indicates the density of the air.
However, upon comparison of the theoretical classifying particle diameter Dp(th) obtained from the said general formula and the classifying particle diameter obtained from actual classifying Dp(obs), it has been found that the Following relationship exists between the two, and these two do not necessarily agree. EQU Dp(obs).gtoreq.Dp(th)
That is to say, the smaller that the target classifying particle diameter becomes, the larger the classifying particle diameter actually obtained Dp(obs) becomes as compared to the theoretical classifying particle diameter Dp(th).
This inventor has found the following to be true, upon studying the cause of the said relationship between the particule diameter Dp(th) and the particule diameter Dp(obs).
As shown in FIG. 6, the tangential direction flow speed distribution of the flow within the vortex-type pneumatic classifier which is provided with guide vanes A8 and vortex flow adjusting vanes (rotor blades) A6 which are opposed across the classifying chamber A7 is described as W in FIG. 6. The classifying particle diameter Dp is determined by the balance between; centrifugal forces F.sub.cA and F.sub.cB which are dependent on tangential direction flow speeds V.sub.tA and V.sub.tB, and drag forces F.sub.dA and F.sub.dB which are dependent on inwardly flowing air speed.
This classifying particle diameter Dp gradually becomes smaller upon the radius which extends from the guide vane part A to the vortex adjusting vane tip part B, and becomes larger again on the inside of the vortex adjusting vane tip.
Therefore, of the classifying material placed between the guide vanes A8 and the vortex flow adjusting vanes A6, the particles which are larger than the classifying particle diameter at point B are recovered to the coarse powder side, while the particles which are smaller than this are recovered to the fine powder side. That is to say that the classifying particle diameter for this machine is the classifying particle diameter D.sub.pB at point B.
As mentioned above, the classifying particle diameter D.sub.pB is determined by the tangential direction flow speed V.sub.tB and inwardly flowing air speed at this point, the actual tangential direction flow speed V.sub.tB does not necessarily agree with the rotor peripheral speed, but has a slight delay. That is to say, the flow speed of the tangential direction flow speed distribution W at point B is slower than the rotor peripheral speed R indicated by the broken line.
On the other hand. V.sub.tB uses the rotor peripheral speed R for calculation of the theoretical classifying particle diameter Dp(th). This is the reason for the difference between the theoretical classifying particle diameter Dp(th) and the actual classifying particle diameter Dp(obs). Especially, in instances where the rotor peripheral speed is great, the difference between the tangential direction flow speed and that of the guide vane part becomes great, and then sufficient acceleration does not occur in this space, so that this tendency becomes prominent. As clearly shown from the said, desired classifying at a desired classifying point cannot be executed by making use of a general formula.
Also, with a conventional vortex pneumatic classifier, the classifying raw material is supplied from the upper portion, and enters the classifying chamber while being dispersed by dispersion plates. On the other hand, the air necessary for classifying is pulled in between guide vanes secured and arrayed around the entire perimeter of the classifier by a fan to the rear of the classifier.
At this point, the classifying air begins homogeneous vortex action as a result of these guide vanes, and is further accelerated by the rotor blades (vortex flow adjusting blades) to the speed necessary for classifying.
That is to say, if the space between the guide vanes and the rotor blades is defined as classifying space, the air flow within that space can be considered to be a two-dimensional vortex flow.
Particles supplied to the classifying space begin vortex action with this vortex flow, and are classified by the balance between centrifugal force and drag force acting upon the particles.
As a result, particles smaller than the classifying particle diameter determined by the balance between the two said forces enter into the interior of the rotor, and are discharged and gathered passing through a discharge duct.
On the other hand, large particles fall by gravity while repeatedly receiving classifying action, and are discharged from a coarse powder discharge duct.
Further, control of the classifying particle diameter is performed by rotor rotational speed or classifying air flow rate, i.e., the centrifugal force or the drag force, acting upon the particles.
Also, in order to perform fine powder classifying, it is necessary to provide great centrifugal force to the particles, and it is necessary to increase the rotational speed of the rotor blades to this end.
However, increasing the said rotational speed causes pressure loss of the said vortex pneumatic classifier owing to circling and turbulence of the air necessary for classifying, necessitating increasing the capacity of the fan providing suction of air. At this time, in the event that there is delay of the air flow as compared to the speed of the rotor blades as said, it becomes necessary to provide extra rotation to the rotor in order to conduct the targeted classifying, and the pressure loss is further increased.
This results in facilities and investments which are overly great, and creates great problems concerning conservation of resource energy. Classifying of powder material such as cement falls in the category of fine powder classifying, and is a relatively coarse classifying of such. Therefore, pressure loss is relatively low, but there is great production volume involved with this sort of powder material, and the proportion of energy costs against the powder material price is of a great proportion, so that the effects of even a small decrease in pressure are great.
In light of the said conditions, this invention aims at classifying granular or powdered material at the desired classifying point not only easy but also accurate.
Another object is to attempt to decrease pressure loss.